In certain forms of medical diagnostic testing a patient is caused to inhale air mixed with gaseous additives introduced by the tester. Then, as part of the test, the composition of the gaseous mixture exhaled by the patient is analyzed, especially with respect to the concentration of the added components. The levels of the additives in the exhaled breath of the patient may be indicative of a variety of conditions, for example, the amounts absorbed into the patient's bloodstream. It is therefore desirable to include as part of the testing apparatus, means for determining and indicating the precise interval during which exhalation occurs in order that the exhaled breath may be sampled for analysis.
In a particular application in use with computerized tomography (CT), a technique has been developed in recent years involving the inhalation of a xenon gas mixture by the patient undergoing a series of CT scans. By measuring the concentration of xenon in the patient's blood and relating this informatin to data derived from the CT scans, images mapping the level and efficiency of cerebral blood flow are generated on the CT scanner. The preferred method for doing so involves measuring the concentration of xenon in the final, or end-tidal, portion of the patient's exhaled breath, which is known empirically to be representative of the concentration of xenon in the bloodstream.
One technique by which exhalation may be sensed is by determining when inhalation commences. Thus, upon an indication of the commencement of inhalation, the test apparatus may be triggered to sample and analyze the gaseous mixture flowing at the instant as representative of the last portion of the patient's exhaled breath. Alternatively, where the test apparatus is continuously sampling and analyzing the gaseous flow, an indication of the commencement of inhalation provides a point in time prior to which, during a given interval, the sampling/analysis process was being performed on exhaled gases.
Various techniques for detecting a patient's inhalation are known in the art. One such technique includes placing electrodes on the patient's chest and sensing a change in electrode conductivity when the chest expands as the patient takes a breath. A disadvantage of this detection technique is its inherent invasiveness, i.e., the necessity for physical attachment of the electrodes to the patient. Also, the required wire connections of the electrodes create a nuisance to both patient and tester.
Another known detection technique takes advantage of the fact that the patient must breathe through a mask assembly in order to inhale the gaseous mixture. By placing a magnetic check valve in the patient's breathing path, e.g., inside the tube which supplies the gaseous mixture to the mask, a small amount of vacuum must be drawn when the patient inhales in order to pull the check valve open. When the vacuum is sensed, e.g., by using a typical off-the-shelf vacuum switch, it indicates commencement of inhalation. One disadvantage of the last-recited detection technique is that it requires the introduction of a restriction into the patient's inhalation path in order to create the vacuum. A second disadvantage lies in the necessity for making adjustments when this detection technique is applied to a variety of patients having a wide range of breathing depths. Thus, without adjustment, a small child may be incapable of drawing sufficient vacuum on a valve set for use by an adult.
Still another detection technique known in the art also entails the use of a magnetic check valve. However, instead of using a vacuum switch to indicate inhalation, the device includes coils of wire wrapped around the valve. The coils generate a pulse when a magnet inside the valve moves in relation to the coils during inhalation. Such a detection technique likewise requires the introduction of a restriction into the patient's inhalation path. A further disadvantage resides in the difficulties and expense experienced in manufacturing valves on a large scale where each valve must be wound with a wire coil.